The present invention generally relates to an improved method of making biomedical ocular devices from a monomer mixture containing hydrophilic and hydrophobic components. By filtering the monomer mixture, the hydrophilic and hydrophobic components are finely dispersed within the mixture. Additionally, the presence of microbubbles is decreased. This method is useful in preparing contact lenses, intraocular lenses and ocular devices such as corneal rings, for example.
Biomedical ocular devices are made from a variety of materials. In general, there are three classes of materials which are used to prepare contact lenses, intraocular lenses, or ocular devices: hydrogels, non-hydrophilic soft materials, and rigid gas permeable materials.
A common type of material for use as a soft contact lens is a hydrogel (see for example, U.S. Pat. No. 3,220,960 to Drahoslav and Wichterle). Hydrogels constitute crosslinked polymeric systems that can absorb and retain water in an equilibrium state. Hydrogels generally have a water content greater than about 5 weight percent and more commonly between about 10 to about 80 weight percent.
A particular group of hydrogel materials include those containing silicone monomers. Polymeric silicone materials are generally hydrophobic and have been used in a variety of biomedical applications, including contact lenses and intraocular lenses. Silicone-containing monomers in contact lenses are desired to increase the oxygen permeability or DK value. Such materials are usually prepared by polymerizing a mixture containing at least one silicone-containing monomer and at least one hydrophilic monomer.
Hydrophobic materials, including silicone-containing monomers, have poor compatibility with hydrophilic monomers. Poor compatibility may result in phase separation where the components of the mixture may actually separate out upon standing or not be uniformly dispersed throughout the solution. In extreme cases, aggregation of the components may result in separate layers. Poor compatibility of the components may result in opaque materials upon curing. This can be particularly problematic when preparing monomer mixtures to be used to make contact lenses since it is necessary that the contact lens be optically clear.
Diluents have typically been used to overcome this incompatibility. A diluent is defined as a substance which is substantially nonreactive with the components in the monomer mixture. Diluents may be aqueous or organic in nature.
In addition to a uniformly dispersed mixture, the monomer mixture must be free of contaminants and microbubbles to ensure the formation of a transparent lens upon curing. The monomer mixture may need to be filtered to remove any contaminants from the initial monomers. The presence of contaminants will affect the transparency of the cured lens, resulting in poor optical quality for the wearer. U.S. Pat. No. 5,789,464 to Muller discloses purifying prepolymers by precipitation with acetone, dialysis or ultrafiltration, with ultrafiltration being especially preferred. Continuously purifying the solution by ultrafiltration results in a selected degree of purity, which can be as high as desired. Pure prepolymers or comonomers result in monomer mixtures free of contaminants resulting in good quality of optical lenses.
Dissolved gases may cause microbubbles in the resultant cured product which may also affect the optical quality and transparency of the cured lens. The dissolved gases are oxygen and nitrogen from the air and may interfere with polymerization. Microbubbles may form in the monomer liquid upon standing or as the monomer mixture progresses through a manufacturing line toward a casting machine.
The monomer mixture may be subjected to a vacuum to remove any dissolved gases in the monomer mixture. Unfortunately, vacuum may also evaporate volatile components from the monomer mixture. U.S. Pats. No. 5,435,943 (Adams et al); U.S. Pat. No. 5,656,208 (Martin et al); U.S. Pat. No. 5,753,150 (Martin et al) and U.S. Pat. No. 5,804,107 (Martin et al) disclose filtering and degassing the monomer mixture. In these patents, the monomer mixture is placed in a drum and drawn by the action of a pump. The monomer passes through a filter in order to remove extraneous particulate contaminates that may be present in the monomer. The monomer then proceeds through a separate degassing unit. The degassing unit consists of a monomer in a gas permeable line, which is surrounded by a chamber. Contained within the gas permeable line is a static flow mixer which causes a turbulent flow and the boundary layer to be broken up. The chamber is maintained at a subatmospheric pressure. Under such conditions, the gases within the monomer mixture are removed. The process of first filtering the monomer mixture and then degassing involves multiple stages.
Non-homogeneous dispersement of monomer components and dissolved gases result in poor optical quality of cured contact lenses, intraocular devices, and ocular devices including corneal rings. The present invention provides for a simple method which improves the optical quality of such lenses and devices.
The present invention is an improved method for producing biomedical ocular devices including contact lenses, intraocular lenses and devices such as corneal rings. The biomedical ocular devices are made from a monomer mixture comprised of a hydrophilic monomer and a hydrophobic monomer. By filtering the monomer mixture through an in-line filter prior to casting, the hydrophilic and hydrophobic monomers are finely dispersed. Additionally, the dissolved gases are broken down, thereby reducing the formation of undesirably large bubbles in the finished biomedical ocular device. The filtration leads to a more homogeneous mixture thereby improving the optical clarity of the finished biomedical ocular device.